how does tan 110 equal tan290

2 answers

110x(π/180)= 0.611π
290x(π/180)= 1.611π
1.611π-0.611π= 1
If difference is one, than it is one rotation around the circle

Dont forget about your unit circle
actually, 180° is 1/2 rotation around the circle.
But tanx has a period of π, which is why the two are equal.